Immunotherapy is already an often suggested treatment modality in the field of cancer therapy. With the targeting possibility of antibodies or members of a specific binding pair a much more precise localization of the active compounds can be achieved, while at the same time the overall dose of the active compound can be lowered, thereby reducing the general detrimental effects. In early attempts (monoclonal) antibodies or other targeting moieties have been loaded directly with radioactive elements such as .sup.67 Ga, .sup.131 I, .sup.99m Tc, .sup.111 In or other isotopes (e.g. Marchalonis J.J., Biochem. J. 113, 299-305, 1969). In a later study coupling of isotopes to antibodies has been achieved by using chelating agents such as EDTA (Krejcarek and Tucker, Biochem. Biophys. Res. Commun. 77, 581-585, 1977) or DTPA (U.S. Pat. No. 4,454,106).
The use of drugs or toxins has also been suggested. The coupling of drugs or toxins to the antibody or to a carrier loaded with one or more antibodies has to be performed through a linking agent, although it is also possible to make recombinant fusion proteins of toxins and/or carriers and targeting moiety.
Linking agents are well known and a considerable range of these reagents is available. In broad terms, a linking reagent comprises two or more reactive functional groups covalently linked together, such as carbohydroxy-, amino-, thio- or sulfhydryl-groups. The covalent linkage between the two functional groups may be direct, but in many cases the reactive functional groups are separated by respective covalent attachment to a bridging group or spacer. The reactive functional groups may be the same or different. Different groups are to be preferred because they allow a more controlled coupling.
The chemical structure of the linker determines the ability of the active compound to be released and to express its activity at the target site. Initially, peptidic linker structures were applied, which were susceptible to cleavage by lysosomal enzymes (Trouet et al., Proc. Natl. Acad. Sci. 79, 626-629, 1982). This approach requires internalization of the conjugates following binding to the target cell. In the target cell the lysosomal enzymes release the active compound from the targeting molecule.
Another development is a linker structure based on aconitic acid, in order to attach amino group containing drugs through an acid labile amide bond (Shen and Ryser, Biochem. Biophys. Res. Comm. 102, 1048-1054, 1981). Here again effective release of the drug requires transport of the conjugates to intracellular, acidic organelles such as endosomes and lysosomes (De Duve et al., Eur. J. Biochem. 137, 391-397, 1983).
The same holds true for U.S. Pat. No. 4,618,492 which describes the use of amino-sulfhydryl linking reagents characterised by the ability to hydrolyse in mildly acidic solutions.
For the application of such a linker, the conjugate has to be internalized. However, only a minor part of the antibodies produced against tumour-associated antigens are able to induce internalization of the immune complex and thus of the active compound attached. This is especially true when the size of the conjugate is increased by the presence of a carrier.
This failure to induce internalization is a major drawback in the use of immunoconjugates in the field of cancer therapy.